Method of crystallization

ABSTRACT

The present invention proposes a crystallization process comprising the steps of feeding a slurry solution in which a terephthalic acid solution or a part of the terephthalic acid is precipitated to a crystallization vessel, precipitating the terephthalic acid in the crystallization vessel, generating a slurry containing crystals and solvent, discharging the slurry, and further comprising the steps of introducing the slurry in a tangential the direction of a hydrocyclone, returning the slurry flowing out as a downward swirling flow from a lower part of the hydrocyclone to the vessel, and discharging the slurry overflowing from the upper part of the hydrocyclone. According to the present invention, crystals of large particle size is obtained and a stable driving ranging long term can be achieved because any adhesion or accumulation of the terephthalic acid crystals does not appear at all in the crystallization vessel.

TECHNICAL FIELD

[0001] The present invention relates to a crystallization process, inwhich a slurry solution of terephthalic acid is fed to a crystallizationvessel, where the terephthalic acid is precipitated and the slurryincluding crystals and a solvent (mother liquor) is discharged. Moreparticularly, the present invention is employed for separation, etc. ofpurified terephthalic acid crystals.

BACKGROUND ART

[0002] The purified terephthalic acid is produced by oxidizingparaxylene with a molecular oxygen in liquid phase, dissolving a crudeproduct of terephthalic acid in hot water, and catalyticallyhydrogenating the aqueous solution of the crude terephthalic acid bycontacting with a catalyst of noble metal belonging to the group VIII ofthe Periodic table in the presence of hydrogen. This production processemploying an apparatus of commercial scale has a production performanceranging many years.

[0003] In this process, the slurry prepared by cooling the hydrogenatedreaction liquid and including the precipitated terephthalic acidcrystals is separated to the first crystallization products and thefirst mother liquor product usually at the temperature of from 120° C.to 220° C. The first mother liquor product contains considerable amountsof dissolved impurities such as paratoluic acid being a hydrogenatedproduct of 4-carboxybenzaldehyde (4CBA), which is a main impurity in thecrude terephthalic acid, the terephthalic acid corresponding to thesolubility of the isolation temperature and other impurities, etc.Further, it may contain small amounts of the terephthalic acid crystals.

[0004] Mere disposal of the first mother liquor product significantlyincreases the load of an effluent treatment. Further, it is rationalthat the paratoluic acid and the terephthalic acid are recovered as thesecond crystallization products and reused because the paratoluic acidis a precursor of the terephthalic acid. The production of the purifiedterephthalic acid containing recovery and return of the secondcrystallization products to the liquid phase oxidation process isdescribed in Japanese Examined KOKOKU Patent Publication No. Shou56-35174 and Japanese Laid-Open Patent Publication No. Hei 10-195016,etc.

[0005] Additionally, Japanese Laid-Open Patent Publication No. Hei8-231465 discloses a mother liquor substitution method for dividingterephthalic acid crystal particles instead of using elevated pressurecentrifugal separation method.

[0006] Regarding the recovery of the second crystallization products tothe liquid phase oxidation process, the foregoing Japanese ExaminedKOKOKU Patent Publication No. Shou 56-35174 further teaches that “Thecrystallization temperature of the second crystallization productsshould be 130° C. to 100° C.”. Further, Japanese Laid-Open PatentPublication No. Hei 10-195016 teaches that “The raw water solventdischarged is subsequently cooled to 40° C. or less by decompression.”.

[0007] According to the experiences of the inventors ranging many years,however, when the slurry with comparatively low concentration of thecrystals is generated in a crystallization vessel as the case ofrecovering the second crystallization products, crystals are easy toaccumulate on the inner wall and the internal structure of thecrystallization vessel. Further, when the concentration of the crystalsin the crystallization vessel is low, a stable separating operation indownstream tends to become difficult in many cases because the particlesizes of the crystals relatively decrease.

[0008] In other words, the inventors tried to introduce the first motherliquor product separated at the temperature of about 145° C. into acrystallization vessel in the above-mentioned purified terephthalic acidpreparing apparatus, to cool down to 100° C. with flash vaporization ofa solvent (water), and to supply the generated slurry to a separatingmeans for the second crystallization products, the internal structure ofthe crystallization vessel was found to be adhered and accumulated bythe crystals. Furthermore, the pressurized filtration method selected asseparating means was found to induce an intense blocking of a filter,and frequent cleanings of the filter with short intervals werenecessary.

[0009] It is an object of the present invention to provide a way of longand stable operation of the crystallization vessel and the separatingmeans of the crystals in the process of feeding the slurry solution ofterephthalic acid to the crystallization vessel, precipitating theterephthalic acid in the crystallization vessel, generating anddischarging the slurry including crystals and solvent.

DISCLOSURE OF THE INVENTION

[0010] As the result of intensive extensive research and investigationabout the approach for recovering the second crystallization products inpurified terephthalic acid preparation apparatus accumulated by thepresent inventors, it has been found that the above-mentioned object isachieved by installing a hydrocyclone between the crystallization vesseland the separating means. It has also been found that, by theinstallation of the hydrocyclone, the particle size of the crystals inthe crystallization vessel increases. Further, by reversing an ordinaryflow in the hydrocyclone, i.e. by reversing the exhaust current thatdischarges a downward swirling flow down to the separating means, and bydischarging the overflow from upper part of the hydrocyclone afterreturning the downward swirling flow to the crystallization vessel,adhesion pile of the crystals on the crystallization vessel is found tobe completely prevented without causing blocking of filter in theseparation of the crystals. Such being the case, the present inventionhas been accomplished on the basis of the foregoing findings andinformation.

[0011] The present invention is a crystallization process comprising thesteps of feeding a slurry solution in which a terephthalic acid solutionor a part of the terephthalic acid is precipitated to a crystallizationvessel, precipitating the terephthalic acid in the crystallizationvessel, generating a slurry containing crystals and solvent, dischargingthe slurry, and further comprising the steps of introducing the slurryin a tangential direction of a hydrocyclone, returning the slurryflowing out as a downward swirling flow from a lower part of thehydrocyclone to the crystallization vessel, and discharging the slurryoverflowing from the upper part of the hydrocyclone.

[0012] Specifically, the present invention is employed for acrystallization process comprising the steps of dissolving a crudeproduct of terephthalic acid obtained by oxidizing paraxylene with amolecular oxygen in liquid phase into hot water, catalyticallyhydrogenating the aqueous solution of the crude terephthalic acid bycontacting with a catalyst of a noble metal belonging to the group VIIIof the Periodic table in the presence of hydrogen, thereafter coolingthe resultant solution, feeding the first mother liquor productcrystallizingly separated most of the terephthalic acid as the firstcrystallization products to the crystallization vessel, supplying theslurry including the second crystallization products obtained in thecrystallization vessel by further cooling the mother liquor product to aseparating means, and recovering the second crystallization products.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a system diagram showing an embodiment of acrystallization process in accordance with the present invention andillustrating a treatment process of the first mother liquor productemployed to the separation for the second crystallization products inthe preparation of a purified terephthalic acid.

EXPLANATIONS OF NUMERICAL SYMBOLS

[0014] In FIG. 1, numerical symbol 1 shows a crystallization vessel,numerical symbol 2 shows a hydrocyclone, numerical symbol 3 shows acrystal separating means, and numerical symbol 5 shows aslurry-circulating pump.

THE MOST PREFERRED EMBODIMENT TO CARRY OUT THE INVENTION

[0015] The following is an explanation of an embodiment of the presentinvention producing a purified terephthalic acid from paraxylene asingredients.

[0016] Liquid phase oxidation reaction of the paraxylene is conducted ina solvent of acetic acid containing water with the existence of a heavymetal and bromine as catalysts. Cobalt and manganese are usuallyemployed as the heavy metal, and there is a case of addition of theother components such as chromium or iron. Air is generally employed asthe oxidizing agent, and an oxygen enriched air obtained by mixingoxygen gas in the air or, on the contrary, the air lowering the oxygenconcentration by mixing nitrogen, etc. is also employed as the oxidizingagent. The reaction temperature is from 170° C. to 220° C. The oxidationreaction may be conducted as a multisteps reaction with two or moresteps. The oxidation reaction products are, essentially, cooled throughseries coupled crystalliser vessel of at least one step, separating theterephthalic acid crystals precipitated, dried by a dryer and providedas a crude terephthalic acid. The crude terephthalic acid is fed into apurification system and purified thereby obtaining a purifiedterephthalic acid.

[0017] In the purification system, the crude terephthalic acid isdissolved in hot water and is supplied to a hydrogenation column as theaqueous solution of 20% to 35%.

[0018] Inside the hydrogenation column, catalysts of noble metalbelonging to the group VIII of the Periodic table are filled andimpurities in the crude terephthalic acid are hydrogenated by supplyinghydrogen. Platinum, palladium, rhodium, ruthenium, etc. are employed asthe noble metal catalyst belonging to the group VIII of the Periodictable, and the catalyst provided by supporting palladium on activatedcarbons is particularly effective. The catalyst blending at least twokinds of the foregoing metals may be employed. Coconut shell charcoal iseffective as the activated carbon.

[0019] Regarding the supplying amounts of the hydrogen, although it isgenerally appropriate to be approximately 2 times of required amountsfor reducing the 4-carboxybenzaldehyde (4CBA) as a main impurity in thecrude terephthalic acid to paratoluic acid, it is necessary to adjustconsidering the amounts of the impurities that can be reduced. Thehydrogenation reaction temperature is in the range of from 250° C. to330° C., and it is particularly suitable to be from 270° C. to 300° C.The hydrogenation reaction time is suitable to be in the range of from 2minutes to 20 minutes.

[0020] The reaction products discharged from the hydrogenation columnare fed to the crystalliser vessel through a filter installedessentially with an object of preventing the flowing out of minutedebris of the catalyst, etc.

[0021] The crystalliser vessel is coupled at least one step in seriesand the filtered reaction products are sequentially cooled in each stepby means of flash vaporization of the water as the solvent, therebyproviding a slurry containing precipitated crystals of the terephthalicacid dissolved in the reaction products.

[0022] After cooling the slurry down to the temperature of from 120° C.to 220° C., preferably to the temperature of from 130° C. to 200° C.,the cooled slurry is supplied to separating means, where the cooledslurry is separated into the first crystallization products and thefirst mother liquor product.

[0023] With regards to the separating means, although various methodsare employed, a centrifugal separation method is adaptive because it hasa practical performance in commercial scale ranging for many years.However, the centrifugal separation method cannot evade the operationunder the elevated pressure because it is the separation method at thetemperature far over the boiling point of the water under the ordinarypressure. The cost required for the investment and maintenance of anelevated pressure centrifugal separator is very expensive.

[0024] As a prior art, Japanese Laid-Open Patent No. Hei 8-231465proposes mother liquor substitution method as alternative for theelevated pressure centrifugal separation method and in this case, rawslurry and fresh water are fed from the upper part and the lower part ofa mother liquor substitution column respectively. Crystals subsidefreely in the mother liquor substitution column and solvent of the rawslurry, the first mother liquor product, is discharged from the top ofthe column, a slurry dissolved in the fresh water as a replaced solventbeing drawn out from the bottom of the column.

[0025] In comparison with the elevated pressure centrifugal separationmethod, the mother liquor substitution method has many advantages inthat investment is markedly small, that it is free from maintenancebecause of having almost no operating mechanism, that the substitutionrate between the mother liquor and the fresh water can be controlledeasily and that it can be driven even at elevated temperature withoutlimitation, etc.

[0026] The first crystallization products of the terephthalic acidobtained by the separating means are dispersed again optionally in thefresh water and crystals containing water are obtained by passing thedispersed solution through the separating means again in the centrifugalseparation method. The crystals containing water are made into purifiedterephthalic acid products by being dried with a dryer. In the motherliquor substitution method, crystals containing water are obtained fromthe slurry in which the first mother liquor product is replaced to freshwater by passing the slurry through the separating means again and theyare made into purified terephthalic acid products by being dried with adryer.

[0027] The first mother liquor product obtained by the separating meansat elevated temperature is fed to the crystallization vessel and iscooled by flash vaporization or other means, and the second slurryincluding the second crystallization products is provided by thecrystallization of the terephthalic acid dissolved in the first motherliquor product. The second slurry is cooled down further on itsnecessity and finally, the second crystallization products are recoveredby passing the slurry through the separating means.

[0028] The gist of the present invention reside in introducing thesecond slurry discharged from the crystallization vessel into tangentialdirection of the hydrocyclone, returning the slurry flowing out as adownward swirling flow from the lower outlet of the hydrocyclone to thecrystallization vessel and supplying the slurry overflowing from theupper outlet of the hydrocyclone to another separating means for thesecond crystallization products.

[0029] In the case where the slurry is directly supplied to theseparating means without passing through the hydrocyclone, a stabledriving ranging long term is difficult, as will be described inComparative Example 1 below, because terephthalic acid crystals adhereand accumulate on the inner wall and internal structure of thecrystallization vessel.

[0030] Further, even in the case where the slurry is supplied to theseparating means passing through the hydrocyclone, the original flow ofthe hydrocyclone supplying the downward swirling flow to the separatingmeans and returning the overflow into the crystallization vessel doesnot show any effects, as will be described in Comparative Example 2below, in the prevention of adhesion and accumulation of theterephthalic acid crystals on the inner wall and internal structure ofthe crystallization vessel.

[0031] In accordance with the present invention, however, by introducingthe slurry drawn out from the crystallization vessel into an inlet ofthe hydrocyclone in the tangential direction of itself, returning theslurry flowing out as a downward swirling flow from the lower outlet ofthe hydrocyclone to the crystallization vessel and discharging theslurry overflowing from the upper outlet of the hydrocyclone, a stabledriving ranging long term can be achieved because any adhesion oraccumulation of the terephthalic acid crystals does not appear at all.Although the reason why the foregoing effect generates is not clear, itis estimated as the following.

[0032] An installation of the hydrocyclone in accordance with thepresent invention remarkably increases the concentration of the slurryin the crystallization vessel. For example, although the concentrationof the slurry in the crystallization vessel was 0.3% or less both inComparative Example 1 without the installation of the hydrocyclone andin Comparative Example 2 that employed the hydrocyclone with originalflow, the concentration of the slurry in the crystallization vessel wasas large as about 10% in Example 1 of the present invention.Additionally, the average particle diameter of the crystal products wasremarkably as large as 120 μm in accordance with the present invention.

[0033] By returning the slurry flowing out as a downward swirling flowfrom the lower outlet of the hydrocyclone to the crystallization vesselin accordance with the present invention, the crystals once adhered tothe inner wall and inside structure of the vessel are presumed to bescraped down by the crystals of large particle diameter induced by theincrease of the concentration of the slurry in the crystallizationvessel.

[0034] According to the present invention, separation of the secondcrystallization products in the latter steps become easy becauseparticle sizes of the crystals in the slurry overflowing from the upperoutlet of the hydrocyclone become extremely large.

[0035] In the present invention, there are not almost any limitation inthe scale or the operating condition, for example, injection angle ofthe slurry, velocity, the flow velocity distribution, etc. in thehydrocyclone. Any adhesion or accumulation of the terephthalic acidcrystals to the inner wall of the crystallization vessel, etc. will beeffectively prevented on the assumption that the function inherent inthe hydrocyclone is achieved to some extent.

[0036] Additionally, a kind of the hydrocyclone is not particularlylimited, and any hydrocyclone of jet flush-in type or Dreasen type bothof general-purpose is used preferably. Further, multi-steps of thehydrocyclone are installed in series.

[0037] The slurry overflowed from the upper outlet of the hydrocycloneis fed to the separating means after further precipitating the dissolvedcomponent by passing the slurry through an additional cooling processdepending on the necessity.

[0038] In the separating means, the slurry is separated into the secondmother liquor product and the second crystallization products by meansof any separating methods of, for example, pressure filtration,centrifugal separation, etc. The second crystallization products aredischarged to a process of reuse, and, as for the second mother liquorproduct, they are usually exhausted outside the apparatus via aneffluent treatment process.

[0039] In the present invention, cooling by flash vaporization ofsolvent is desirable as the method of making the terephthalic acidprecipitate from the slurry.

[0040] The concentration of the crystals in the slurry overflowing fromthe upper outlet of the hydrocyclone is preferably 5% or less, morepreferably 1% or less.

[0041] For a better understanding of the present invention together withother objects, advantages and capabilities thereof, reference is made tothe following description in connection with the drawings. FIG. 1 is anexample of system diagram of a crystallization process according to thepresent invention illustrating a treating process of the first motherliquor product employed to the separation for the second crystallizationproducts in the preparation of purified terephthalic acid.

[0042] In FIG. 1, the first mother liquor product (i.e. a solution or aslurry solution with partially precipitated solute) from the separatingmeans at an elevated temperature is fed to the crystallization vessel 1through feed pipe 4 for the first mother liquor product.

[0043] In crystallization vessel 1, the mother liquor is cooled down byflash vaporization and a slurry containing crystals of crystallizedterephthalic acid is provided. The vapor generated in thecrystallization vessel is exhausted outside the vessel through flashvapor exhaust pipe 10.

[0044] The slurry provided in crystallization vessel 1 is drawn outthrough line 6, and is introduced into introduction inlet 7 intangential direction of hydrocyclone 2 by means of slurry circulatingpump 5.

[0045] The downward swirling flow generated in hydrocyclone 2 isreturned to crystallization vessel 1 through line 8. Rising overflow inthe hydrocyclone is supplied to crystal separating means 3 through line9.

[0046] The general-purpose separating method such as pressure filtrationmethod or centrifugal separation method is employed as crystalseparating means 3, where the slurry is separated into the secondcrystallization products and the second mother liquor product.

[0047] The second crystallization products are drawn out through line 11and are discharged to recycling process. The second mother products aredrawn out through line 12 and are routed to an effluent treatmentprocess.

[0048] The present invention is broadly adaptive for the processaccompanied by crystallization and separation, and makes the crystalseparating operation in the following step easy because the crystalswith a large particle size can be obtained.

[0049] As will be described in the Example below, by introducing theslurry drawn out from the crystallization vessel into an inlet of thehydrocyclone in the tangential direction of itself, returning the slurryflowing out as a downward swirling flow from the lower outlet of thehydrocyclone to the crystallization vessel and discharging the slurryoverflowing from the upper outlet of the hydrocyclone, in accordancewith the present invention, a stable driving ranging long term can beachieved because any adhesion or accumulation of the terephthalic acidcrystals does not appear at all induced by the generation of crystalswith extremely large crystal particle sizes.

EXAMPLES

[0050] In the following examples are described several preferredembodiments to concretely illustrate the invention, however, it is to beunderstood that the invention is not intended to be limited to thespecific embodiments.

Example 1

[0051] In an apparatus for producing purified terephthalic acid bycatalytic hydrogenation process dissolving the crude terephthalic acidobtained by liquid phase oxidation of paraxylene in water, acrystallization operation of the first mother liquor product obtained bymother liquor substitution method was conducted by the use of anapparatus shown as system diagram in FIG. 1.

[0052] By feeding the first mother liquor product with the temperatureof 145° C. obtained by mother liquor substitution method tocrystallization vessel 1 through feed pipe 4 for the first mother liquorproduct, the operation was conducted in accordance with the systemdiagram. The supplying amount of the first mother liquor product tocrystallization vessel 1 was 100 parts of water, 0.3 parts ofterephthalic acid, which partially exist as crystals, respectively.Almost all other impurities were dissolved in water.

[0053] In crystallization vessel 1, the first mother liquor product wascooled down to 100° C. by flash vaporization of water and a slurrycontaining crystals by crystallization of dissolved terephthalic acidwas obtained.

[0054] The slurry was drawn out and introduced into introduction inlet 7in the tangential direction of jet flush-in type hydrocyclone 2 by meansof slurry circulating pump 5. About 33 parts of water and accompaniedcrystals were drawn out through downward swirling flow outlet 8 and werereturned to crystallization vessel 1. From the upper overflow outlet 9,the slurry with the temperature of 100° C. was drawn out. Theconcentration of the crystals among the slurry in line 6 was a littleover 10%. An averaged particle diameter of the crystals was about 120μm.

[0055] The second crystallization products were recovered from theslurry with the temperature of 100° C. flown out from the upper overflowoutlet 9 by means of the alteration operation of two pressure filtersemployed as the separating means and were discharged to oxidationreaction process through line 11. Further, the second mother liquorproduct separated from the slurry were drawn out through line 12 androuted to an effluent treatment process.

[0056] Continuous operation ranging about half a year of the foregoingsteps in succession did not cause any problems at all.

Comparative Example 1

[0057] The operation similar to Example 1 was conducted withoutinstalling hydrocyclone 2. That is, the slurry with the temperature of100° C. was drawn out from crystallization vessel 1 by pump 5 and afterrecovering the second crystallization products by means of thealteration operation of two pressure filters employed as the separatingmeans, the operation discharged the second crystallization products tooxidation reaction process through line 11. Further, the second motherliquor product separated from the slurry were drawn out through line 12and routed to the effluent treatment process.

[0058] At this time, an averaged particle diameter of the crystals inline 6 was about 20 μm. After the continuous operation ranged for 13days, the slurry became impossible to be drawn out from crystallizationvessel 1. By overhaul inspection of crystallization vessel 1, the innerwall and the internal structure of the crystallization vessel were foundto be adhered by the crystals. Further, the crystals accumulated overthe bottom of the vessel. Furthermore, the cleanings of the filters withintervals of about 2 days were necessary for the continuous operation ofthe pressure filters.

Comparative Example 2

[0059] In Example 1, 33 parts of water overflowed from the upperoverflow outlet 9 of jet flush-in type hydrocyclone 2 with theaccompanying crystals were returned to crystallization vessel 1 therebysupplying the slurry drawn out from the downward swirling flow outlet 8to separating means 3 provided in downstream side of crystallizes vessel1. The concentration of the crystals among the slurry in line 6 was alittle over 0.2% and an average particle diameter of the crystals wasabout 70 μm.

[0060] The second crystallization products were recovered from theslurry with the temperature of 100° C. drawn out from the downwardswirling flow outlet 8 by means of the alteration operation of twopressure filters employed as the separating means and were discharged tooxidation reaction process through line 11. Further, the second motherliquor product separated from the slurry was drawn out through line 12and routed to the effluent treatment process.

[0061] After the continuous operation ranged for 11 days, the slurrybecame impossible to be drawn out from crystallization vessel 1. Byoverhaul inspection of crystallization vessel 1, the inner wall and theinternal structure of the crystallizer vessel were found to be adheredby the crystals. Further, the crystals accumulated over the bottom ofthe vessel. Furthermore, the cleanings of the filters with intervals ofabout 2 days were necessary for the continuous operation of the pressurefilters.

What is claimed is:
 1. A crystallization process including the steps offeeding a slurry solution in which a terephthalic acid solution or apart of the terephthalic acid is precipitated to a crystallizationvessel, precipitating the terephthalic acid in the crystallizationvessel, generating a slurry containing crystals and solvent, anddischarging the slurry, characterized in comprising the steps of:introducing the slurry in a tangential direction of a hydrocyclone,returning the slurry flowing out as a downward swirling flow from alower part of the hydrocyclone to the vessel, and discharging the slurryoverflowing from the upper part of the hydrocyclone.
 2. Thecrystallization process according to claim 1, wherein said precipitatingthe terephthalic acid in the crystallization vessel is conducted withcooling by flash vaporization of solvent.
 3. The crystallization processaccording to claim 1 or claim 2, wherein crystals and mother liquorproduct are separated each other after supplying said slurry overflowingfrom the upper part of said hydrocyclone to a separating means.
 4. Thecrystallization process according to any one of claims 1 to 3 whereinthe concentration of crystals in said slurry overflowing from said upperoutlet of said hydrocyclone is 5% or less.
 5. The crystallizationprocess according to any one of claims 1 to 4 further comprising thesteps of: dissolving a crude product of terephthalic acid obtained byoxidizing paraxylene with a molecular oxygen in liquid phase in hotwater, catalytically hydrogenating the aqueous solution of the crudeterephthalic acid by contacting with group VIII noble metal catalyst inthe presence of hydrogen, thereafter cooling the resultant solution,feeding the first mother liquor product crystalizingly separated most ofthe terephthalic acid as the first crystallization products to thecrystallizer vessel, and recovering the second crystallization productsfrom the slurry including the second crystallization products obtainedin the crystallization vessel by further cooling the mother liquor. 6.The crystallization process according to claim 5, wherein said slurrysolution of terephthalic acid including the second crystallizationproducts is provided by mother liquor substitution method.